When a polymeric material is used in food packaging or the like, it is desired to use a material having low gas permeability in order to prevent deterioration of the content. Among polyester resins, polyethylene terephthalate has been frequently used as a material for food packaging in various beverage containers and the like, due to the good balance of moldability, mechanical properties and gas barrier property. However, particularly in food packaging where long-term storage stability is required, even the polyethylene terephthalate resin cannot be said to essentially have a sufficient gas barrier property, and thus a variety of gas barrier property modifiers, for polyester resins or polyethylene terephthalate having excellent gas barrier properties have been suggested.
It has been suggested in JP-B No. 63-40444 that a copolymer of isophthalic acid, ethylene glycol and 1,3-bis(2-hydroxyethoxy)benzene is a resin having low carbon dioxide or oxygen permeability, but this feature does not correspond to a sufficient gas barrier performance.
A study has been also conducted on the use of a polyester copolymerized with an oxycarboxylic acid, such as polyglycolic acid. U.S. Pat. No. 4,565,851 discloses improvement in the gas barrier property by blending polyglycolic acid with polyethylene terephthalate. However, since polyethylene terephthalate is poorly miscible with polyglycolic acid, it is difficult to obtain a transparent resin composition, and thus there is a problem that it is difficult to form a packaging material having good external appearance. Further, JP-A No. 10-138371 discloses a multilayer container made of a thermoplastic resin such as polyethylene terephthalate and polyglycolic acid, and JP-A No. 10-337772 discloses a blow molded container made of polyglycolic acid. However, when polyethylene terephthalate and polyglycolic acid are used to form a multilayer molded container, its poor adhesiveness makes it necessary to use an adhesive for the adherence of interfaces in view of practical utility. Also, there is a problem that polyglycolic acid is likely to undergo hydrolysis and thus, is difficult to be used as a practical packaging material. When the polymers are used in multilayer molded bottles, polyglycolic acid is not highly miscible with polyethylene terephthalate, and thus there are problems that it is not easy to recover and recycle the products. There is also a problem that polyglycolic acid is likely to undergo hydrolysis and thus, is difficult to be used as a packaging material for practical use.
JP-A No. 2003-136657 suggests a laminate container including at least a polyglycolic acid layer and a thermoplastic polyester resin layer. However, since polyglycolic acid is not well miscible with polyethylene terephthalate, when the two polymers are used in laminate molded bottles, the transparency is deteriorated. Moreover, when the laminate is reprocessed by molding, the transparency is reduced, and thus there is a problem that it is difficult to recover and recycle the products.
In addition, polyesters resulting from copolymerization of oxycarboxylic acids and aromatic dicarboxylic acids have been suggested in JP No. 2564469, JP-B No. 7-21107, JP-A No. 61-179226 and the like. JP No. 2564469 and JP-B No. 7-21107 describe the use of a polyethylene terephthalate component and an aliphatic oxycarboxylic acid copolymerized-polyethylene isophthalate component by subjecting the components to multilayer structuring or blending. But, this polyethylene isophthalate containing oxycarboxylic acid does not contain the oxycarboxylic acid in a sufficiently large amount and thus, has insufficient gas barrier property. Furthermore, JP-A No. 61-179226 suggests a copolymer of isophthalic acid and oxycarboxylic acid. However, this copolymer is produced at a high temperature of 260° C. or higher, resulting in a copolymer having low density, and this copolymer cannot be said to have sufficient gas barrier property.